It's not possible yet, but because it works via IMAP if you have a root e-mail account (normally just your username, the password to access a control panel), you can see all your folders in a collapsed tree because they are treat as separate in-boxes. You may have to subscribe to them.
It's not very efficient, but you can set-up multiple identities and forward all your e-mail to one global address which you login to and manage your e-mail from there. However, your e-mail won't get split up into folders or separate like it would if you logged in via root.